The present invention relates to hearing prostheses, and more particularly to an improved technique for programming, or fitting, a cochlear implant system to a particular patient.
Cochlear prostheses produce sensations of sound in deaf patients by direct electrical stimulation of the auditory nerve. In modern, multichannel cochlear prostheses, several different sites are stimulated at various distances along the cochlea to evoke the different pitches of sound perception that are normally encoded by nerve activity originating form the respective sites. The patterns of electrical stimulation are derived from acoustic signals picked up by a microphone and transformed by a so-called speech processor that is programmed to meet the particular requirements of each patient. Several different schemes for processing the acoustic signal and transforming it into electrical stimuli have been developed and are well-described in the scientific literature and various patents. For purposes of the present invention, these schemes—also known as speech processing strategies—can generally be considered as either sequentially, partially-simultaneously or fully-simultaneously speech processing strategies.
The conventional setting of electrical stimulation levels in sound processors for cochlea implant systems—a process generally referred to as “fitting” the speech processor to a patient—has involved the stimulation of single channels (comprised of monopolar or bipolar stimulation pathways) employing stimuli that do not resemble the stimulation patterns inherent in the on-going speech signal. See, e.g., U.S. Pat. No. 5,626,629, incorporated herein by reference. Typically, during such fitting process, gated-bursts of some fixed burst duration and constant amplitude are delivered to the patient. This procedure of obtaining psychophysical measurements is often quite laborious. The patient's task is to set a level where sound is barely audible, and then set a level where sound is comfortably loud.
Disadvantageously, after going through the time-consuming and laborious cochlear-implant-fitting process, when the patient's microphone is enabled and speech stimuli are delivered to all channels, either sequentially, partially-simultaneously or fully-simultaneously, the psychophysically set levels bear little resemblance to the final parameters set in the patient's sound processor. Adjustments to the overall level of stimulation as well as other parameters tend to be required to mold the psychophysically derived parameters into a viable program that appropriately maps the perceived speech stimuli to electrical stimuli that may be delivered directly to the patient's cochlea. Hence, essentially two fitting procedures are typically required—one to set the psychophysical levels, and a second to make adjustments to such levels.
A further complication in setting levels in a sound processor is the fact that in cochlear-implant systems, which employ narrow pulse widths (e.g., 10.7 microseconds) and high rates of stimulation, obtaining single-channel measurements for estimates of comfortable loudness is not practical. At high rates of stimulation, the behavior of the electrically stimulated auditory system can mimic that of the normally healthy ear in that perception of constant amplitude stimuli cannot be maintained over time by all patients. Thus, the need arises, in setting the levels in cochlear implant processors, for using either actual speech stimuli, or stimuli that mimic the nature of speech.